Fluids based on fluorocarbon compounds are widely used in systems of heat transfer by vapour compression, especially air conditioning, heat pump, refrigeration or freezing devices. The common feature of these devices is that they are based on a thermodynamic cycle comprising the vaporization of fluid at low pressure (in which the fluid absorbs heat); compression of the vaporized fluid up to a high pressure; condensation of the vaporized fluid to liquid at high pressure (in which the fluid expels heat); and depressurization of the fluid to complete the cycle.
The choice of a heat-transfer fluid (which may be a pure compound or a mixture of compounds) is dictated, on the one hand, by the thermodynamic properties of the fluid, and, on the other hand, by additional constraints. Thus, a particularly important criterion is that of the impact of the fluid under consideration on the environment. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) have the drawback of damaging the ozone layer. Non-chlorinated compounds such as hydrofluorocarbons, fluoroethers and fluoroolefins are therefore now generally preferred.
Another environmental constraint is that of the global warming potential (GWP). It is therefore essential to develop heat-transfer compositions that have a GWP that is as low as possible and good energy performance.
Moreover, to lubricate the moving parts of the compressor (or compressors) of a vapour compression system, a lubricant oil must be added to the heat-transfer fluid. The oil may generally be mineral or synthetic.
The choice of the lubricant oil is made as a function of the type of compressor, so as not to react with the heat-transfer fluid itself and with the other components present in the system.
For certain heat-transfer systems (especially of small size), the lubricant oil is generally permitted to circulate throughout the circuit, the pipework being designed such that the oil can flow by gravity to the compressor. In other heat-transfer systems (especially of large size), an oil separator is provided immediately after the compressor, as is a device for controlling the oil level, which returns the oil to the compressor(s). Even when an oil separator is present, the pipework of the system must still be designed so that the oil can return by gravity to the oil separator or to the compressor.
Document WO 2004/037 913 describes compositions based on fluoroolefins and especially based on tetrafluoropropene or pentafluoropropene. Example 2 reports the miscibility of 1,2,3,3,3-pentafluoropropene (HFO-1225ye) with various lubricant oils, and also that of 1,3,3,3-tetrafluoropropene (HFO-1234ze) with various lubricant oils. Example 3 reports the compatibility of HFO-1234ze and of 3,3,3-trifluoropropene (HFO-1243zf) with lubricant oils such as polyalkylene glycols.
Document WO 2005/042 663 specifically concerns the miscibility of mixtures of fluoroolefins and of lubricant oils. The examples provided for these mixtures are essentially the same as those of document WO 2004/037 913.
Document WO 2006/094 303 describes a large number of heat-transfer compositions comprising fluoroolefins, and especially 2,3,3,3-tetrafluoropropene (HFO-1234yf), and additional compounds. Moreover, the document generally teaches combining the list of the numerous possible refrigerant mixtures with a list of lubricant oils.
Document WO 2007/126 414 describes a large number of mixtures of heat-transfer compounds, and especially mixtures comprising 2,3,3,3-tetrafluoropropene (HFO-1234yf) and ammonia. The document also teaches the addition of any lubricant chosen from a list of conventional lubricants.
Documents WO 2008/009 928 and WO 2008/009 922 describe heat-transfer compositions based on pentafluoropropene, tetrafluoropropene and at least one additional compound, which may be ammonia.
Document US 2006/0 243 945 describes a large number of mixtures of heat-transfer compounds, and especially quaternary compositions based on HFO-1234yf, ammonia, difluoromethane (HFC-32) and trifluoroiodomethane. A generic list of possible lubricants is cited.
When the heat-transfer compound(s) are of poor miscibility with the lubricant oil, this oil tends to be trapped in the evaporator and not return to the compressor, which prevents correct functioning of the system.
In this regard, there is still a need to develop heat-transfer compositions with a low GWP (which have good energy performance), in which the heat-transfer compounds show good miscibility with the lubricant oil.
In particular, HFO-1234yf is a heat-transfer compound that is very advantageous especially on account of its low GWP and its good energy performance. On the other hand, its miscibility with certain lubricant oils such as polyalkylene glycol oils is imperfect and limits its application. It is thus desirable to improve the miscibility of compositions based on HFO-1234yf with the usual lubricant oils.